In a gas turbine engine, pressurized air is provided from the compressor stage to the combustor, whereupon it is mixed with fuel and is burned in the combustion chamber. The amount of pressurized air which enters the fuel/air mixers. and correspondingly the inner and outer passages of the combustor, has typically been regulated by inner and outer cowls located upstream of the fuel/air mixers and the combustor dome. Such cowls have been generally held in place by means of a bolted joint which includes the combustor dome, the cowl, and either the inner or outer combustor liner. Accordingly, both the outer and inner cowls of a gas turbine engine experience a slight change in pressure thereacross, as well as a vibratory load induced by the engine. While these environmental factors have a greater effect on the outer cowl, they nevertheless cause wear on both cowls and consequently limit the life thereof.
In addressing this problem, the prior art has generally taken one of two approaches. The first of which involves use of a sheet metal body for the cowls with a lip formed at the leading edge thereof, preferably by curling or wrapping the sheet metal around a damper wire. However, it has been found that this design is life-limited due to a rubbing-type wear occurring at the interface of the wire and the sheet metal body caused by a thermal mismatch between the wire and the wrap. More specifically, the thermal mismatch causes the sheet metal to unwrap around the wire. creating a gap between the wire and the cowl. In addition, white noise exiting the diffuser and/or combustor acoustics create high cycle fatigue vibratory loading of the wire against the sheet metal wrap. Thus, the combined rubbing and vibratory induced shaking of the wire against the metal wrap result in the wrapped portion of the cowl thinning, cracking and eventually liberating sheet metal and wire fragments. Failures of cowls having this design have been found to occur at substantially less than the cowl HCF life requirement for the applicable engine.
Another cowl design involves a machined ring which forms the leading edge lip of the cowl, where the ring is welded to a formed sheet metal body. Such a machined ring provides a solid lip for the cowl, which is desirable, but circumferential welding thereof to the formed sheet metal body has resulted in stress concentrations both in and around the weld which are sources of failure initiation of the cowl. Yet another one-piece cowl design is disclosed in a U.S. patent application entitled "One-Piece Combustor Cowl" and having Ser. No. 08/811,754 now, U.S. Pat. No. 5,924,288, to Fortuna, which discloses a cowl which is casted that has a solid lip of increased thickness at a leading edge thereof. while suitable for its intended purpose, this cowl tends to be both heavier and more costly than a sheet metal cowl.
Accordingly, it is desirable for a one-piece gas turbine engine cowl to be developed for use with combustors which is able to sustain the stress levels imposed thereon for a desirable number of hours without succumbing to high cycle fatigue and still direct air flow to the combustor in a manner consistent with the requirements of the fuel/air mixers and the inner/outer passages. It is also desirable for such a cowl to be both lightweight and inexpensive in terms of materials, processing and specific fuel consumption.